1. Field of the Invention
The present invention relates to a semiconductor device having an insulated gate transistor, e.g., a MOS transistor and, more particularly, to a semiconductor device with improved current drivability.
2. Description of the Related Art
A conventional MOS-transistor structure will be described with reference to FIGS. 8 and 9. FIG. 9 is a plan view of the conventional MOS transistor and FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 9.
In FIG. 8, an isolation region 102 is formed on the major surface of a p-type substrate 101. In regions isolated by the isolation region are formed n-type source/drain regions 103.sub.1 and 103.sub.2. A gate insulating layer 104 is formed on the channel region between the source/drain regions 103.sub.1 and 103.sub.2. A gate electrode 105 is formed on the gate insulating layer 104.
As shown in FIG. 9, the source/drain regions 103.sub.1 and 103.sub.2 are formed on both sides of the gate electrode 105. The width L of the gate electrode 105 is called the gate length and the width W of each of the source/drain regions 103.sub.1 and 103.sub.2 in the direction normal to the direction of the channel is called the channel width.
Under the condition that Vd&gt;V.sub.G -V.sub.T the current drivability Id of the MOS transistor is expressed by EQU Id=(W/2L).times..mu.Cox (V.sub.G -V.sub.T).sup.2 ( 1)
where L stands for the channel length, W stands for the channel width, .mu. stands for mobility, Cox stands for capacitance of the gate insulating layer, and Vd, V.sub.G and V.sub.T stand for a drain voltage, a gate voltage and a gate threshold voltage, respectively.
To produce a MOS transistor requiring great current drivability Id, the channel width W in expression (1) is currently made large. With such an approach, however, an increase of the channel width W results in an increase of the area of the MOS structure, thus making a fine structure of the device difficult.
For this reason, a transistor has been developed in which a pedestal region is provided between source and drain regions to protrude from a semiconductor substrate and a gate electrode is formed on side walls of the pedestal region. This transistor is useful for making the device structure fine, but still it does not have sufficient current drivability.